netdev-dpdk: Fix memory leak in dpdk_do_tx_copy().

netdev-dpdk: Fix memory leak in dpdk_do_tx_copy().

This patch fixes a bug where rte_pktmbuf_alloc() would fail and
packets which succeeded to allocate memory with rte_pktmbuf_alloc()
would not be sent and leak memory.

Also, as a byproduct of using a local variable to record dropped
packets, this reduces the locking of the netdev's mutex when
multiple packets are dropped in dpdk_do_tx_copy().

Signed-off-by: Ryan Wilson <wryan@nicira.com>
Acked-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>